1. Field of the Invention
The present invention relates to a motor control device which controls a synchronous motor including a reluctance motor with a high performance, and, in particular, relates to a motor control device which performs the control with a sensorless manner.
2. Conventional Art
In order to control such as speed and torque of a synchronous motor, it is necessary to detect or estimate its magnetic pole position. Thus, such as speed and torque of the synchronous motor can be controlled by performing a current control or a voltage control thereof based on the detected magnetic pole position.
Recently, methods of controlling a synchronous motor with no magnetic pole position sensor has been proposed.
A first method thereof is disclosed, for example, in JP-A-7-245981 (1995) and in a paper No. 170 at Heisei 8th National Meeting of Industrial Application Division for Japan Electrical Engineering""s Society, which relates to a method of estimating the magnetic pole position based on a parallel motor current component and an orthogonal motor current component (current components in rotary coordinate system) in response to AC voltage application, and is characterized in that the detection of a magnetic pole position at a standstill and during a low speed rotation of the motor can be realized without using a magnetic pole position sensor.
A second method of superposing an additional voltage is disclosed, for example, in JP-A-11-150983 (1999) and JP-A-11-69884 (1999) in which method an additional voltage is applied so as to prevent magnetic flux saturation even in a high torque region, thereby, sensorless detection of a magnetic pole position is realized in a range from a low load to a heavy load at a standstill or during a low speed rotation.
Further, a third method is, for example, disclosed in JP-A-8-205578 (1996) in which a salient pole property of a synchronous motor is detected from a correlation between a voltage vector applied to the synchronous motor through a pulse width modulation control (PWM control) and a motor current ripple component (in vector amount) corresponding thereto. Further, since the third method utilizes usual PWM signals for controlling the synchronous motor, an advantage is obtained that no additional signals for the detection is required.
However, when detecting the magnetic pole position with the first method while driving the motor, it is necessary to extract a current having the same frequency component as that of an AC voltage used for the detection through such as a band pass filter using a notch filter and a Fourier integration. In particular, when the motor rpm increases, separation between motor input frequencies and AC voltage frequencies used for the detection becomes difficult, thus, a problem is posed that a stable motor drive at a high rpm is difficult. Further, the method requires a measure so as not to be affected by the switching characteristic of the invertor concerned. Namely, in contrast to the carrier wave frequencies of PWM signals which are from several kHz to 20 kHz, the frequencies of the AC voltage used for the detection are low at about several 100 Hz, therefore, noises of several 100 Hz may be generated when the motor is driving.
The second method is intended to improve performance when the motor is driven from a standstill condition or at a low speed rotating condition, however, the timing of current detection and the relation with the PWM signals which become important for a high speed drive of the motor are not disclosed as well as no measures for realizing the position detection with a high accuracy are disclosed.
Further, in order to realize the third method it is necessary to detect correlation between the motor current condition and the applied voltage every time when the PWM signals vary. Namely, it is the base requirement that the detection of the motor current condition and recognition of the applied voltage condition have to be performed at least six times for every one cycle of the carrier waves which requires a high performance controller.
A first object of the present invention is to provide a motor control device which estimates a magnetic detection of motor current over a broad range from a standstill condition to a high speed rotating condition while using a non-expensive controller and controls a synchronous motor including a reluctance motor with a high response characteristic.
A second object of the present invention is to provide a high performance motor control device which can compensate a current follow-up property based on counter electromotive force information even if the motor speed varies.
According to the present invention, a motor control device is provided with an AC motor, an electric power inverter which applies a voltage to the AC motor and a control unit which controls the applied voltage with PWM signals in synchronism with carrier waves, wherein a magnetic pole position on a rotor of the AC motor is estimated through detection of current in the AC motor in synchronism with the carrier waves. For example, in a synchronous motor having a salient characteristic a current in the motor is detected while varying the applied voltage for every half cycle of the carrier waves and a current difference vector (a vector in static coordinate system) for the every half cycle is determined. Subsequently difference of twice current difference vectors (hereinbelow called as current difference difference vector) and difference corresponding to twice applied voltage vectors (hereinbelow called as voltage difference vector) are calculated, and the applied voltage (voltage difference vector) is controlled so that the phase difference thereof assumes 0. When the phase difference is rendered 0 with the above method, the phase of the voltage difference vector assumes the direction (d-axis) of the magnetic pole position, thereby, a magnetic pole position sensorless control can be realized. Since the change of the applied voltage is performed for every half cycle of the carrier waves, only the phase of the PWM signals is shifted which prevents generation of noises. Further, since the current is detected in synchronism with the carrier waves, the average values of the applied voltages for respective phases are accurately recognized which provides a characteristic that the relation between the applied voltage and the current difference can be detected in a short time represented by the carrier wave cycle. For this reason, the present invention is effective for sensorless approach in a field of motor control requiring a high response property.
Further, through the use of information on the current difference vector the counter electromotive force of the synchronous motor can be detected accurately. By making use of the thus detected counter electromotive force a counter electromotive force compensation in the current control system is effected, thereby, a control system having a good current follow-up property is constituted in comparison with the conventional compensation of counter electromotive force which was estimated from the speed.
Through the detection of current in synchronism with the carrier wave cycle, the inventers found out a variety of methods of detecting the magnetic pole position other than the above explained which will be explained below in detail.